1. Field of the Invention
This invention is useful in the underground mining of minerals and particularly, the mining of coal. While the invention is especially applicable in coal mining operations, and will be described in that environment, the scope of the invention should not be limited to that type of operation.
2. Description of the Invention
Generally, in a coal mining operation, coal is removed from the face of a coal seam and deposited onto a flexible conveyor for removal from the mine.
Throughout the mine there are passageways which permit miners to move around through the mine. These passageways are separated by pillars which are areas of the mine that have not been mined. As these pillars have exposed faces, the vertical forces on the pillars cause the pillars to expand into the passageways. If the expansion is severe, the pillar side may collapse into the passageways.
In order to keep the pillar side from collapsing, angled rib bolts are used to support the exposed sides of the pillars in the mine. These bolts are inserted into long holes drilled into the face of the pillar, usually at a 45.degree. angle with respect to the face of the pillar. The bolts are held in place by an expansion shell or set resin in which one end of the bolt is anchored.
In the past, a crescent washer at the head or exposed end of the bolt held a header or bearing plate against the face of the pillar to inhibit expansion of the pillar side and prevent its collapse. One of the problems which occurred in the prior art structure was the breakage of the rib bolt when the stress of the expanding pillar exceeded the failure strength of the rib bolt.